1. Field of the invention
The present invention relates to an inductive load drive circuit and in particular to a drive circuit which is coupled with an inductive load such as an ignition system of a vehicle for generating a high voltage.
2. Prior Art
As shown in FIG. 1, such a conventional drive circuit 300 is connected with a controller 100 comprising a current source 101 and a switch 102 via a harness 103 and comprises an NPN power transistor 1 having an emitter which is grounded, a series-connected resistors 3 and 4 having an intermediate connection which is connected with the base of the power transistor 1 and a Zener diode 2 interposed between the resistor 3 and the collector of the power transistor 1. The resistor 4 is provided for preventing any malfunction due to leaked current. The collector of the power transistor 1 is connected with the primary coil of an ignition coil 202 which is an inductive load 200. The secondary coil of the ignition coil 202 is connected with an ignition plug (not shown).
Operation of this prior art drive circuit will be described with reference to FIG. 2A. When the switch 102 is closed at the time t1, a drive current is supplied to the power transistor 1 from the current source 101 via the switch 102 and the harness 103. Accordingly, a collector current of the power transistor 1 increases for a period of time T1 since the time t1. When the switch 102 is turned off at the time t2, the collector current suddenly descreases for a period of time T2. The collector current never flows for a period of time T3 following the period of time T2.
The period of time T2 is a period of time for which the power transistor 1 is brought into a completely non-conductive state after it has been brought into a conductive state once since the switch 102 is turned off. In response to a rapid change in the collector current, a high collector voltage of the power transistor 1 is generated across the primary coil of the ignition coil 202.
A waveform of the collector voltage for the period of time T2 is shown in detail in FIG. 2B. When the collector voltage abruptly rises up to exceed the voltage V.sub.z of on of the Zener diode 2, a feed back current IFB is supplied to the base of the power transistor 1 from the Zener diode 2 via the resistor 3. The feed back current turns the power transistor 1 on so that the collector voltage settles to a constant value V.sub.CL. The voltage value V.sub.CL is expressed as follows: EQU V.sub.CL =VZ+R3.times.IFB
wherein R3 represents the resistance of the resistor 3.
In order to generate a high voltage across the primary coil of the ignition coil, the power transistor 1 is turned on or off for a short period of time T2 in the prior inductive load drive circuit. The collector current increases to an overvoltage Vp which is higher than the stable voltage V.sub.CL for a period of time .tau. since the time t2. This is due to a fact that the feed back current charges the distributed capacitance on the base line of the power transistor 1 (for example, the base parasitic capacitance of the power transistor 1, parasitic capacitance of the harness 103, etc.) so that the power transistor 1 is maintained non-conductive for a period of time .tau.. The collector voltage increases to an overvoltage in accordance with a relation L.times.d (collector current)/dt.
Therefore, there has been a problem that as the controller 100 is so remote from the drive circuit 300 that the length of the harness 103 is long, the period of time .notident. is extended and the overvoltage Vp increases to such a value that it breaks the power transistor 1.